The Subsea Passive Heave Compensator (SPHC) is an installation tool designed to compensate vertical heave during sensitive installation of subsea equipment in an offshore environment. The vertical heave source is typically generated by an installation vessel's motion and or crane tip motion. The SPHC is designed to operate in air or in water at depths up to 10,000 ft. The SPHC is an inline tool that uses the principles of spring isolation to generate a net heave compensation effect or spring isolation effect. The tool is a nitrogen over oil spring dampening device. For spring isolation to occur, the natural period of the spring/mass system must be increased to a ratio higher than the forcing/heave period. Spring isolation begins to occurs when the natural period of a system is 1.414 times greater than the forcing/heave period.
Prior art heave compensators use spring isolation theory and hydraulic spring dampers do exist. The difficulties with these types of compensators are the effect that hydrostatic pressure has on the units. Further, hydrostatic pressure limits the ability to soften the spring system to achieve greater spring isolation. The limits imposed by depth effect are primarily the sensitivity to external pressure. The flatter the spring curve, the more sensitive it is to external pressure and the greater chance that errors in mass calculations can render the heave compensator useless. The hydrostatic pressure has a net effect on the piston rod calculated by the hydrostatic pressure times the piston rod area. This net load compresses the rod as the compensator is lowered to depth.
The novel design of the SPHC is the use of pressure balancing to mitigate/eliminate the depth effect. A compensating cylinder is added to the tool to eliminate the depth effect. The compensating cylinder uses area ratio's to provide a precise amount of back pressure on the low pressure side of the hydraulic cylinder to offset the load from the high pressure cylinder rod caused by hydrostatic pressure. FIG. 3 shows one prior art solution to external pressure with the use of a tail rod. The tail rod exerts an equal force as the piston rod and for this reason eliminates the depth effect. However, the length of the unit is doubled. Length is considered a constraint for handling purposes and the tail rod method is not considered ideal. Using the compensator cylinder with the heave compensator allows for a depth compensation to occur without adding to the length of the unit. With depth compensation, the volume of nitrogen can be increased to lengthen the natural period greater than when using a system without compensation.